It was discovered that PHP incorrectly handled certain character sequences
when applying HTTP response-splitting protection. A remote attacker could
create a specially-crafted URL and inject arbitrary headers.
(CVE-2011-1398, CVE-2012-4388)

It was discovered that PHP incorrectly handled directories with a large
number of files. This could allow a remote attacker to execute arbitrary
code with the privileges of the web server, or to perform a denial of
service. (CVE-2012-2688)

It was discovered that PHP incorrectly parsed certain PDO prepared
statements. A remote attacker could use this flaw to cause PHP to crash,
leading to a denial of service. (CVE-2012-3450)

Update instructions:

The problem can be corrected by updating your system to the following
package versions:

Ubuntu 12.04 LTS:
php5 5.3.10-1ubuntu3.4

Ubuntu 11.10:
php5 5.3.6-13ubuntu3.9

Ubuntu 11.04:
php5 5.3.5-1ubuntu7.11

Ubuntu 10.04 LTS:
php5 5.3.2-1ubuntu4.18

Ubuntu 8.04 LTS:
php5 5.2.4-2ubuntu5.26

In general, a standard system update will make all the necessary changes.